The present invention relates to an electronic sensor for brake systems in vehicles, in particular to an electronic sensor for vehicles with so-called anti-lock brake systems. As is well-known, the braking force in such brake systems is modulated when brakes are applied too strongly and there is a risk that the wheel slip (the ratio between the peripheral velocity of the wheel and the vehicle velocity) increases to such an extent that reduced braking efficiency and ability to exert lateral force cause loss of control of the vehicle.
A large number of wheel sensors have been developed, many of which use digital electronic techniques and similar control philosophies. A common, and dominating, problem is to find a reasonable solution to the requirement that the sensor receive information regarding the momentarily prevailing frictional condition between the vehicle wheels and a road surface. In previously known sensors this is accomplished by a continuous calculation of the retardation and acceleration of the wheel with the aid of a microcomputer in which are stored reference values to which the retardation is continuously compared. On the basis of these comparisons, the sensor provides a series of different signals instructing the brake force modulator of the system to reduce the brake force, to hold it constant or to re-apply the brake force with different application rates depending on the values of the calculated wheel accelerations. The known sensors generally comprise a quartz clock that continuously provides a pulse train of high frequency, for instance one pulse each fifth millisecond. The sensor provides signals with a duration comprising multiples of 5 milliseconds, which is the basis for the ability of the sensors to provide, depending on the calculated retardations and accelerations, instructions to the brake force modulator and, during a certain number of periods of for instance 5 milliseconds, to reduce, hold constant, or re-apply brake force.
In this way the sensor can be made to provide a signal pattern that adapts the brake modulator's control of the brake force to the momentarily prevailing state of the road in an acceptable way. While such a sensor will operate acceptably, such a complex control system can only be accomplished with modern, very advanced microelectronics of superior component quality, such as that known to experts in the field as "military specified".
Such sensors are very expensive and sensitive to functional disturbances due to their complexity and sensitivity to external disturbances. The remaining electrical systems of the vehicle are a disturbing interference source which can be reasonably protected against by interference surpressing measures on the vehicle. External interference sources over which the vehicle manufacturer has no control, for instance in the form of other vehicles, traffic-lights, welding machines, radar and TV equipment, power transmission lines, thunderstorms and the extremely high and low temperatures that have to be considered in vehicle environments, are much harder to deal with. Further negative cost and reliability factors for such complex systems are that the accompanying complex brake force modulator and extensive cabling introduce an interference threat per se by "antenna action".